The overall objective of the proposed research is to determine whether or not there is some deficit in the ability of alcoholics to respond to the interoceptive cues which are assumed to be consequences of changes in blood alcohol level. The main issue to be dealt with is the effect of behavioral tolerance on the ability of drinkers to discriminate changes in their blood alcohol levels. A second objective is to determine the relationship between one's current blood alcohol level and one's discrimination ability. In the first phase of the study the behavioral tolerance of subjects will be determined by observing the effect of alcohol ingestion on performance on a match to sample operant task. In the second phase of the study, the effects of behavioral tolerance on discrimination ability will be assessed using a psychophysical approach: subjects will simply be asked to report when they perceive a change in their interoceptive sensations. Data obtained from the proposed study should answer the question as to whether drinkers who have acquired a behavioral tolerance to alcohol are less able to perceive changes in their blood alcohol level than are drinkers who have not acquired such tolerance. The data obtained should have some important implications for the feasibility of relying on blood alcohol level discrimination training as the basis for training programs in controlled drinking.